The present invention relates to a sheet carrier apparatus for conveying a sheet at a constant speed by using a roller and, more particularly, to a sheet carrier apparatus in which a stencil sheet is conveyed by driving a platen roller to rotate while being pressed against the platen roller by a thermal head.
In a conventional perforating machine, a stencil sheet is conveyed by driving a platen roller to rotate while being pressed against the platen roller by a thermal head.
The platen roller is required to contact the stencil sheet closely with heating elements of the thermal head; thus, the platen roller is made of an elastic material such as rubber, thereby ensuring an enough nip area where the stencil sheet is pinched between the thermal head and the platen roller. In this machine, expansion or shrinkage of the stencil sheet in the conveying direction is to be determined by a diameter of the platen roller and a feeding speed by a motor as driving means.
Since the platen roller is made of rubber as explained, changes of temperature or working conditions of the platen roller vary a diameter of the platen roller.
A change in the diameter of the roller leads to a change in a conveying speed of the roller although a rotating speed of the roller remains unchanged. Thus, expansion/shrinkage ratio of a perforated sheet changes, thereby deteriorating a dimensional accuracy of an image on a printed sheet.
The present invention is achieved to solve the problem described above and; therefore, an object of the present invention is to provide a sheet carriage apparatus which provides a constant conveying speed without being badly affected by changes in working condition thereof.
A sheet carrier apparatus as defined in the first aspect of the present invention comprises a roller having a cylindrical circumferential surface made of an elastic material, a temperature sensor for detecting a temperature of the roller, and processing means for setting a rotational speed of the roller based on a detection signal from the temperature sensor.
A sheet carrier apparatus as defined in the second aspect of the present invention comprises a thermal head; a platen roller having a cylindrical circumferential surface made of an elastic material and pressed against the thermal head, the platen roller being rotated to convey the stencil sheet between the platen roller and the thermal head; a temperature sensor for detecting a temperature of the platen roller; and processing means for setting a rotational speed of the platen roller based on a detection signal from the temperature sensor.
In a sheet carrier apparatus as defined in the third aspect of the present invention, the processing means is so constituted that the processing means executes a predetermined computing equation based on the detection signal from the temperature sensor, thereby obtaining a diameter of the roller in the detected temperature, and outputs a control signal for setting the rotational speed of the roller according to the diameter, so that the sheet can be conveyed at a constant speed regardless of a temperature change in the sheet carrier apparatus as defined in the first aspect.
In a sheet carrier apparatus as defined in the fourth aspect of the present invention, the computing equation is as follows:
St=Ss÷{1+xcex1(txe2x88x92T)÷xcfx86s}
where, St is a corrected rotational speed [rpm] of the platen roller; Ss is a rotational speed [rpm] of the platen roller in T xc2x0 C.; xcfx86s is a diameter [mm] of the platen roller in T xc2x0 C.; xcex1 is an expansion coefficient [mm/xc2x0 C.] of the platen roller; and t is a temperature[xc2x0 C.] of the platen roller, in the sheet carrier apparatus as defined in the third aspect.
In a sheet carrier apparatus as defined in the fifth aspect of the present invention, an xcex1 in a temperature range over a predetermined temperature is smaller than an xcex1 in a temperature range less than the predetermined temperature in the sheet carrier apparatus as defined in the fourth aspect.
A sheet carrier apparatus as defined in the sixth aspect of the present invention further comprises a memory which prestores data indicating a rotational speed corresponding to the temperature of the roller, in which apparatus the processing means reads the memory based on the temperature indicated by the detection signal from the temperature sensor so as to obtain the rotational speed corresponding to the temperature, thereby controlling rotation of the roller in the sheet carrier apparatus as defined in the first aspect.
In a sheet carrier apparatus as defined in the seventh aspect of the present invention, the temperature sensor directly detects the temperature of the roller while being in contact with the roller in the sheet carrier apparatus as defined in the first aspect.
In a sheet carrier apparatus as defined in the eight aspect of the present invention, the temperature sensor is disposed at a predetermined distance with the roller and detects a peripheral temperature of the roller in the sheet carrier apparatus as defined in the first aspect.
A temperature sensor detects temperature of a roller conveying a sheet, and the detected temperature is outputted to processing means. The processing means calculates a diameter of the roller based on the detected temperature, thereby determining a rotational speed for a constant circumferential speed of the roller based on the diameter.
The roller, while being controlled to rotate at the determined rotational speed, conveys the sheet at a constant speed regardless of the changes in temperature.
The expression xe2x80x9ctemperature of the rollerxe2x80x9d as used herein means not only the temperature of the roller itself but also a peripheral temperature around the roller or a temperature of an atmosphere in which the roller is disposed.
In the present invention, at least a cylindrical circumferential surface of the roller is made of an elastic material; otherwise, the whole roller may be made of the material.